1. Field of the Invention
The subject invention is concerned with the formation of thin wall cellular plastic containers such as plastic drinking cups. In manufacturing the containers, expandable resin beads, usually polystyrene beads, are introduced into the space between the male and female mold members. Steam may then be introduced into the cavity between members through internal steam ports in the male mold member to expand the beads and cause the beads to fuse forming the container as the end product. The mold members are then separated, and the container is ejected by a blast of air through air orifices in the male mold member.
The small beads are of small size and comprise a significant amount of very fine particles. The beads and particles upon heating can adhere and clog the ports and orifices, preventing the subsequent introduction of fluids into the mold cavity. It is therefore necessary to provide some means to protect the ports, while allowing for the exiting of the fluid from the port into the mold cavity.
One effort has been directed to the use of an elastomeric ring, which fits snugly against the port, temporarily blocking the ports during the filling of the mold cavity with the beads. Upon applying fluid pressure from the ports, the elastomeric ring expands allowing the passage of fluid. After a relatively short period of time, the elastomeric ring loses its elasticity or wears out, requiring frequent replacement. In addition, the escape of fluids, particularly vapors, about the elastomeric ring causes a characteristic high decibel shrieking noise, which is particularly annoying and is subject to prohibition or elimination under safety standards. It is therefore desirable to find an alternative means which prevents the ports from clogging while minimizing high decibel noise and provides for relatively infrequent changes of the protective mechanism.
2. Brief Description of the Prior Art
U.S. Pat. No. 3,825,378 describes a mold having an annular groove employing an elastomeric ring as a check valve. This patent also refers to the relevant prior art.